The present invention relates to a method for transmitting signals in a wireless communication system and to a wireless communication system enabling an improved beam steering algorithm for non-line of sight (NLOS) wireless systems.
Wireless communication is used in a large variety of technical fields, such as mobile telephone, wireless LAN, walkie-talkies, broadcast radio systems, point-to-point radio systems and many other known and future applications. The communication radius covered by a respective wireless communication system basically depends on the technique used. Whereas cellular communication systems, such as the GSM and the UMTS system, are adapted for a communication radius up to about 10 km (or more), wireless LAN is in the range of about 100 m (or more) and the Bluetooth system is in the range of several 10 m (or more). The major influences on the communication range of a wireless communication system are the radio frequency and output power used. Although only little absorption of electromagnetic waves in the atmosphere occurs at the radio frequency used for GSM and UMTS, a significant absorption occurs in the 60 GHz range, which makes it quite well suited for low range and indoor wireless communication. Furthermore, the kind of transmission and/or reception antennas used for respective wireless communication technique varies depending on a respective field of application. E.g., if a number of receivers has to be reached or if the location of the receivers is unknown or varies frequently, e.g. due to movement, wide beam antennas or omni-directional antennas are sometimes used. However, the utilisation of wide beam antennas in high data rate mm waves wireless communication systems is problematic, because of the multi-path fading effect.
As shown in FIG. 3 according to prior art, when wide beam antennas 24, 25 are used for both transmitter and receiver sides and the line of sight link P0 is blocked by an obstacle 22, there exist a lot of reflections paths P1, P2 between the transmitter 20 and the receiver 21, i.e. transmission paths in which the transmitted electromagnetic wave is reflected at least once by objects 23a, 23b, 23c, 23d before it reaches the receiver. The channel delay spread might be over tens of symbol periods when the data rate is high, e.g. over 1 Gbps, which leads to severe inter-symbol interference due to deep frequency selective fading.
Two conventional solutions exist for such kind of non line of sight (NLOS) user scenarios, where both of the solutions need high-speed and complex signal processing circuits. One solution adopts a channel equaliser including linear, decision feedback or maximum likelihood sequence estimation (MLSE) equaliser. When the channel delay spread is much longer than the symbol duration, the equaliser becomes complex and needs a lot of processing power. Another solution is the Orthogonal Frequency Division Multiplexing (OFDM) technique, which is already adopted in wireless LAN systems. However, due to its inherent linear modulation and high peak to average ratio problems, the power consumption of the power amplifier (PA) in such systems is very high. Obviously, a high speed fast Fourier transformation (FFT) and other signal processing modules are required for demodulating a 1 Gbps signal. Therefore, it is important to find other solutions which do not require complex and high speed baseband circuitry for high data rate millimeter wave range communication systems.
Document EP 1 659 813 A1 proposes to use a pair of sharp beam steering antennas on the transmitter and on the receiver side. The narrow beam antennas are able to be steered into different positions and thereby the transmitter and the receiver are adapted to establish a first communication path for wireless communication via those narrow beam antennas. Further, document EP 1 659 813 A1 proposes that the transmitter and the receiver are adapted to automatically establish at least one alternative communication path for wireless communication via said first and second narrow beam antennas, such alternative communication path being spatially different from said first communication path.
The disadvantage with the beam steering algorithm of the state of art is that in order to find the optimum steering position and thereby the optimum transmission path, it is necessary to estimate the channel quality of each possible transmission path. This is e.g. achieved by measuring bit error rate (BER) performance. In order to achieve a reliable estimation of the channel quality elaborated, complex channel quality measurements are necessary. On the other hand, if the complexity of the channel quality measurement is reduced, the risk of false estimations of channel quality increases.